Current State of the Art of Externally-Thread Implants
Implants with threads over a substantial portion of their external surface, designed for self-tapping insertion, are generally known and commercially available. These screw implants usually have an unthreaded, annular portion at the proximal end of the implant that is commonly referred to as the neck portion, with the remainder of the external surface substantially threaded to or near to the distal end of the implant. The self-tapping features located in the threaded surface near the distal end of the implant are the subject of several patents (Linkow U.S. Pat. No. 4,713,004; Jorneus U. S. Pat. No. 5,269,685). These features serve the function of cutting the threads in the cylindrical passage formed in the jawbone tissue of a person to receive the implant as the externally-threaded implant is rotated to a depth that places the neck of the implant above, at or just below the crest of the jawbone.
Self-tapping insertion of an externally-threaded implant is accomplished by forming, as by machining, one or more grooves on the sidewall extending upwardly from the distal end parallel to the longitudinal axis of the dental implant and through at least one full diameter external thread. These grooves create cutting edges that function to scrape off bone chips during threading of the implant into the cylindrical hole prepared in the bone tissue. The grooves also provide cavities with adequate volume to contain the bone tissue material to allow full seating of the implant.
Some self-tapping implants also provide a through-hole connecting two channels on opposite sides of the implant to provide additional cavity space to harbor bone chips and to further stabilize the implant once bone regeneration has occurred. Self-tapping insertion of the implant has proven advantageous from a time-saving standpoint (Fribert B. et al.; JOMI 1992; 1:80-84) by reducing surgical time by 3 minutes or more per implant. Self-tapping insertion of the externally-threaded implants also improves the initial stability needed for direct bone attachment following a healing period, referred to as osseointegration, by creating a more intimate contact with the bone than placement following use of a bone tap surgical instrument. This more intimate initial fit has also been demonstrated to result in an increased percentage of bone attachment to the implant surface after healing (Cook S. et al. J Oral Implant 1993: 4:288-294). For self-tapping insertion to be effective in dense bone, the cutting edges created by the grooves through the distal threads must be sharp enough to shave bone chips. Roughening the implant surface by grit-blasting, or by grit-blasting followed by coating the surface of the implant with a spray of molten titanium called Titanium Plasma Spray (TPS) or coating the surface with a bio-reactive material such as Hydroxylapatite (HA), rounds these cutting edges, decreasing the cutting efficiency of the self-tapping features. This can necessitate increasing the torque forces needed to insert the self-tapping implant in dense bone to the point that damage may occur to the wrench-engaging feature in the proximal portion of the implant, resulting in failure to seat the implant fully in the bone chamber.
Self-tapping screw implants are usually machined from a biocompatible metal of suitable strength such as commercially-pure titanium or from medical grade titanium alloy. The selection of Grade 1 or 2 commercially-pure (CP) titanium, with tensile strengths lower than Grade 3 or 4 CP titanium or titanium alloy (6AI/4V), may preclude the incorporation of through-holes because of the lower tensile strength. Such lower tensile strength may also limit the density of bone that the implant can self-tap into because of the lower resistance to distortion of the wrench-engaging surfaces at or near the proximal end of the implant as higher torque forces are required to cut through dense bone.
Some self-tapping screw implants are sold with a machined surface (Nobelpharma and Implant Innovations, Inc. implants) and others (Core-Vent Corporation's SCREW-VENT, SWEDE-VENT and CORE-VENT implants) are further treated after machining by washing in dilute HF acid to remove loose titanium particles and other contaminants. Acid etching creates pits on the surface of the implant, increasing the surface roughness, compared to the untreated machined surface, as measured by the average distance between the peaks and valleys created on the surface in the form of machining grooves or etch pits. Some commercially available self-tapping screw implants have their threaded external surfaces treated to increase surface roughness while maintaining the neck portion relatively smooth by leaving it with a machined or etched surface or by mechanically polishing the surface. The texture of the implant's external surface is increased in roughness by grit-blasting with a variety of bio-compatible particles such as titanium oxide (Astra implants) or aluminum oxide (CORE-VENT implant, pre-1986). The degree of roughness can be varied by varying the size of the abrasive particles and by varying the force and the duration of the blasting procedure. Some screw implants, after machining, are grit-blasted to roughen the surface preparatory to applying a coating of either Titanium Plasma Spray (TPS: Straumann's implant), which provides both a rough and porous surface, or a coating of a bio-active material such as Hydroxylapatite (HA: STERI-OSS, SCREW-VENT and SWEDE-VENT implants). HA may be densely applied and of high crystallinity, which produces a surface roughness approximating that of acid etching or less dense and/or less crystalline which produces a surface roughness that could match or exceed that of TPS coating or grit-blasting.